Monday, September 10, 2018

What's the world record look like for RPMs? One of the fastest-spinning objects in the world is a tiny, levitating dumbbell created by a team of American and Chinese researchers—a nanoscale rotor that can spin more than a sixty billion times in just one minute.

Tongcang Li and Jonghoon Ahn have levitated a nanoparticle in vacuum and driven it to rotate at high speed, which they hope will help them study quantum mechanics and the properties of the vacuum. Image Credit: Purdue University/Vincent Walter.

In 2013, scientists at the University of St. Andrews made headlines by propelling a tiny sphere to a record-breaking 600 million rpm (revolutions per minute). This new object out-spins that by a factor of 100. To put these numbers in context, a vinyl LPs rotates at 33.33 rpm, the tires on a car traveling at 60 mph rotate at 840 rpm, and the fans in a fighter jet's engine rotate at around 20,000 rpm. 60 billion rpm is fast.

The research behind this spinning dumbbell could help us design experiments to probe two very different, hard-to-study areas—the properties of materials in extreme conditions and the quantum nature of gravity and empty space. The research was published in July, in the American Physical Society’s journal Physical Review Letters, by a collaboration of researchers from Purdue University, Peking University, and Tsinghua University.

We don’t usually think of light as pushing or pulling on objects, but it can, and the strength and direction of the force depends on the properties of the light. Under the right conditions, a focused laser beam can trap, move, levitate, and even rotate a tiny particle—abilities that have earned such beams the name "optical tweezers". In the St. Andrews experiment, the team used optical tweezers to rotate a tiny sphere of calcium carbonate inside of a vacuum chamber, where it’s not slowed down by constant collisions with air particles.

This new research was led by Tongcang Li from Purdue University. “In 2015, we wanted to drive nanoparticles to rotate ultrafast,” he says, so they set up an experiment similar to the one performed by the St. Andrews team. In place of a sphere, the Purdue group used a diamond nanoparticle. There was just one problem—it didn’t work. Instead of rapidly spinning, the diamond nanoparticle vibrated in low vacuum. At higher vacuum, it got lost.

To study the vibration in more detail, the team decided to do experiments on nanoparticles under higher vacuum conditions. This required switching from diamond to pure silica nanoparticles. One day in 2016, Li’s graduate student Jonghoon Ahn was playing around with this new setup when he observed something surprising—the ultrafast rotation they initially set out to create! At first, the team didn’t even know the shape of the rotating particle. It took them more than a year to understand Ahn’s observation and reproduce particles with that shape.

The rotating “thing” was two silica nanoparticles so close together that that they formed a dumbbell-shaped object. This was surprising and a bit mystifying, but eventually the researchers pieced together the big picture. “Now we understand the system much better,” says Li. In fact, team can make a levitated nanodumbbell vibrate or rotate by changing a property of the laser called its polarization.

Further exploration of these two situations—a vibrating dumbbell and an ultrafast rotating dumbbell—could yield deep insights into the quantum world. We often think of a vacuum as being completely empty, but in reality pairs of virtual particles constantly flit in and out of existence. Theories predict that these virtual particles can have a real, although tiny, influence the motion of real particles. With this research, we'll hopefully be able to get a better understanding of these tiny forces—and maybe even observe them.

Furthermore, the vibrating dumbbell situation provides a surprising analogy to a historic 18th-century physics experiment called the Cavendish experiment, in which the physicist Henry Cavendish determined key values that describe the strength of gravity, and its relation to mass. This nano-version of his experiment is an important step toward probing the quantum nature of gravity, say the researchers.

In a completely different application, you can also use ultrafast rotation to study materials. Like just about anything, if you get a nano-dumbbell spinning too fast, it will fall apart. The point at which this happens depends on the experimental settings, like vacuum pressure and object size, and on the properties of the material. If we set up an experiment and let a dumbbell spin until it falls apart, we can learn a lot about how the material handles high stress.

So, there you have it—the story of a chance discovery of ultrafast rotation that enables us to push materials to their absolute limits, and that could bring us closer to understanding the quantum weirdness of our universe. It’s enough to make your head spin.